The degree and nature of the interaction of low megahertz ultrasound and mammalian tissues is studied. As ultrasound is now widely employed clinically in diagnosis and therapy, the opportunity for reproductive organs to become exposed, to significant levels for appreciable periods of time, is extant. The mammalian ovary has been selected for investigation in the present study, which was initiated several years ago. The study involves determining threshold levels at which functional and/or structural changes occur to the murine corpora lutea, including determination of absorption coefficients. The effect of multiple exposures is also assessed. The investigation includes electron microscopy and fertility studies, The results of these studies will provide bases for assessing risk incurred by ovarian tissues during exposure to clinical ultrasound at therapeutic and at diagnostic levels and regimes.